Pyrolytic carbon or pyrocarbon has long been deposited by the thermal decomposition of a gaseous or vaporized hydrocarbon, or some other carbonaceous substance in vaporous form, in the presence of a substrate on which the deposition occurs. It has also been known for a number of years that the character of the pyrocarbon deposited is highly influenced by the amount of surface area available for deposition within a fluidized bed of a certain unit volume and that accordingly the characteristics of the pyrocarbon actually deposited can be desirably influenced by careful control of the bed size, as generally taught in U.S. Pat. No. 3,399,969 to Bokros, et al. This patent points out that the pyrocarbon coating of relatively large objects, such as objects having a dimension of about 5 millimeters or greater, in the presence of an ancillary bed of small particles (i.e. of a size measured in tens or hundreds of microns), is best controlled by controlling the available deposition surface area relative to the volume of the coating enclosure wherein the actual deposition is taking place.
In U.S. Pat. No. 3,977,896 to Bokros and Akins, an improved process was described and illustrated for depositing pyrolytic carbon coatings of substantial thickness which would have a very uniform crystallinity throughout the entire thickness of the pyrocarbon deposited. Such uniformity was achieved by maintaining the total deposition surface area within the coating enclosure relatively constant by adding particles of small size to the coater while coated particles (which have grown in size) are being removed from the enclosure at a controlled rate by regulating the rate of flow of an inert gas stream upward through a conduit through which the particles exit.
Subsequent to the issuance of the '896 patent, U.S. Pat. No. 4,546,012 issued in the name of Brooks which discloses an improved fluidized bed apparatus for coating such relatively large objects in association with a bed of particles being levitated, wherein a constant bed size is maintained from a volumetric standpoint by employing a weir tube having a spillover entrance hole to define the maximum upper level of the fluidized bed by its location at a desired vertical level within the coating enclosure. Preferably the weir tube is closed at its top and has its spillover hole facing away from the centerline of the coating enclosure. Sufficient purge flow of inert gas upward through the tube is maintained so as to prevent any substantial quantity of dust from gravitating down the tube with the coated particles being withdrawn. U.S. Pat. No. 4,594,270 also issued in the name of Brooks and shows an apparatus for removing particles from a desired level within such a fluidized bed wherein a larger object is being levitated and coated by pyrolytic decomposition. This patent teaches the employment of a vertically slidable sampling tube, preferably controlled from the exterior of the furnace, which can be located so as to have its opening at a desired vertical level from which the sample is to be withdrawn. Flow of inert gas through the sampling tube is controlled so as to selectively allow particles to be withdrawn from the bed when the tube is at the desired vertical level.
Although the foregoing patents disclose acceptable processes for controlling pyrolytic deposition within a fluidized bed system, it is extremely difficult, if not impossible, to determine the actual size of the bed surface area at any given moment, and as coating processes become more sophisticated, there is a desire to be able to even more precisely control the characteristics of the pyrocarbon being deposited. As a result, more improved methods of control have been sought.